Patent application title: Purification of glycerin obtained as a bioproduct from the transesterification of triglycerides in the synthesis of biofuel

Abstract:

Methods for purifying glycerin contaminated with one or more lower boiling
alcohols such as methanol, ethanol, straight, branched or cyclic C3-C6
alcohols, and the like. The methods are particularly useful for purifying
crude glycerin phases recovered from the synthesis of biofuels. The
present invention uses distillation techniques to strip alcohol
contaminants from glycerin. In contrast to conventional methods that
carry out distillation either under substantially anhydrous or very wet
conditions, the present invention carries out distillation in the
presence of a limited amount of water, e.g., from about 0.8 to about 5
parts by weight of water per 100 parts by weight of contaminated glycerin
to be purified.

Claims:

1: A method of purifying crude glycerin, comprising the steps of:a)
providing an alkaline admixture comprising glycerin, soap, a fatty acid
ester, and at least one other alcohol, said other alcohol having a lower
boiling point than glycerin;b) adding a sufficient amount of water so
that the admixture includes from about 0.8 to about 5 parts by weight
water per 100 parts by weight of the admixture;c) after adding the water,
distilling the admixture under conditions effective to strip away
substantially all of the other alcohol;d) after stripping the other
alcohol, lowering the pH of the admixture with aqueous acid under
conditions effective to convert the soap to free fatty acid and to form a
first organic phase comprising the free fatty acid and the fatty acid
ester and a second aqueous phase comprising the glycerin; ande)
separating the organic and aqueous phases.

2: The method of claim 1, wherein step (b) comprises adding from 1 to 4.5
parts by weight of water per 100 parts by weight of the admixture.

3: The method of claim 1, wherein the other alcohol is selected from
methanol, ethanol, and linear, branched or cyclic C3 to C6 alcohols.

6: The method of claim 1, wherein the aqueous acid is selected from HCl,
H2SO4, phosphoric, and citric acid.

7: The method of any of claim 1, wherein step (b) comprises lowering the
pH to a value in the range from about 2 to about 4.

8: The method of claim 1, further comprising the step of adding an
antifoaming agent to the admixture while step (c) is being carried out.

9: The method of claim 1, wherein the distillation occurs at a pressure of
about 760 mmHg.

10: The method of claim 1, wherein enough water is added so that the
distillation occurs at a temperature of less than 200.degree. C. in order
to reduce the amount of the other alcohol to less than 1 weight percent.

11: The method of claim 10, wherein enough water is added so that the
distillation occurs at a temperature of less than 190.degree. C. in order
to reduce the amount of the other alcohol to less than 1 weight percent.

12: The method of claim 1, wherein the admixture has an alkaline pH during
at least a portion of step (c).

13: The method of claim 12, wherein the pH is 10-13.

14: The method of claim 13, wherein the pH is about 12.

15: The method of claim 1, wherein the admixture of step (a) is
substantially anhydrous.

Description:

FIELD OF THE INVENTION

[0001]The present invention relates to the purification of crude glycerin.
More particularly, the present invention relates to the purification of
crude glycerin recovered from the transesterification of triglycerides in
the synthesis of biofuels.

BACKGROUND OF THE INVENTION

[0002]Biodiesel is a type of biofuel that is manufactured from
triglycerides, diglycerides, and monoglycerides, but predominantly
triglycerides. Vegetable oils, nut oils, animal fats, seed oils, fish
oils, and the like are examples of suitable feedstocks containing
triglycerides. In a typical synthesis, triglycerides are subjected to a
transesterification reaction between the triglyceride and a
stoichiometric excess of a suitable alcohol such as methanol, ethanol, or
other linear, branched or cyclic C4, C5, or C6 alcohols. Use of ethanol
and methanol are most common. The reaction occurs in the presence of a
base catalyst and usually under substantially anhydrous conditions in
which water is excluded as much as is practical. The reaction may be
carried out in continuous or batch equipment.

[0003]The desired product of the transesterification is a fatty acid
ester. When the alcohol reactant is methanol, this product is referred to
as a fatty acid methyl ester, or FAME. The final contents of the reaction
stream will also include glycerin (also known as glycerol) as a
by-product alcohol; unreacted excess reactant alcohol; residual and spent
catalyst (the spent catalyst may be present as a soap depending upon the
catalyst used); and soaps present from fatty acids or other impurities
that might have been present in the oil feedstock. The reaction usually
proceeds far enough to completion that the amount of glyceride (whether
mono, di, or tri) is de minimis.

[0004]The by-product glycerin is insoluble in the product ester to a large
degree. Accordingly, the reaction stream separates into two phases as the
transesterification reaction progresses. One phase is relatively rich in
the fatty acid ester, while the other phase is relatively rich in
glycerin. All of the constituents of the reaction vessel tend to be
distributed among both phases, however. The glycerin layer is referred to
herein as "crude glycerin". The other organic ingredients of the crude
glycerin layer are referred to herein as contaminants with respect to the
crude glycerin.

[0005]Glycerin itself is a triol having the formula
HOCH2CH(OH)CH2OH and has many uses. By way of example, it is
used in medical and nutriceutical preparations, in personal care
products, in foods and beverages, in animal feed, as a raw material to
manufacture other compounds such as polyols and polyurethanes, in surface
coatings and paints, in making absolute ethanol, in textiles, in de-icing
fluids, in softeners and surfactants, in antifreeze, and the like.
Accordingly, it is highly desirably to purify the crude glycerin inasmuch
as glycerin has so many product uses. The methanol, fatty acid, and fatty
acid ester contaminants in the crude glycerin also are valuable materials
and are desirably recycled as well. For instance, the methanol, fatty
acid, and the fatty acid ester can be recycled for use in further
synthesis of biofuel or other products.

[0006]A key step in the purification of crude glycerin involves stripping
the methanol from the crude glycerin using distillation techniques.
Conventional methodologies have been problematic, however. In some
instances, the distillation occurs under substantially anhydrous
conditions. However, relatively high bottom temperatures must be used,
e.g. temperatures above about 200° C., even above about
210° C., and even above about 220° C., in order to reduce
the methanol content of the crude glycerin to acceptably low levels when
distillation is anhydrous. At these temperatures, there is a substantial
tendency for undue amounts of polyglycerin to form, undermining the goal
to obtain purified glycerin. Temperature reduction by operation under
vacuum to lower the temperatures requires a more sophisticated
condenser/cooling system.

[0007]Carrying out wet distillation, however, is also problematic. Often,
decanted wash water might be added to the crude glycerin in order to
recover more fatty acid ester in an organic phase, which segregates as an
upper layer on top of the glycerin. Methanol stripping from this or any
other similarly wet layer is difficult due to excessive foaming caused by
soap that is present. There is too much water, soap, and foaming for
anti-foaming agents to help control this in any effective manner.

[0008]To attempt to avoid foaming, the crude, wet glycerin can be
acidified to lower the pH to a value such as 2 to 5 in order to convert
the soap into fatty acid. Still, the stripping of methanol from such
acidic glycerin poses serious challenges due to corrosivity and reboiler
plugging issues. First, the crude glycerin is corrosive due to its low
pH, requiring equipment with expensive metallurgy for proper handling.
Reboiler plugging can be caused by salts and the high distillation
efforts to separate a dry methanol from such a wet glycerin. Reboiler
plugging is a severe economic issue. The heat transfer coefficient
decreases and the unit loses production capacity over time. Eventually,
the unit will have to be shut down to remove salts by washing them out,
by hydro blasting, or other suitable technique.

[0010]There remains a strong need for effective methodologies that can
purify crude glycerin, including aspects of this purification that
involve separating crude glycerin from other alcohol contaminants such as
methanol.

SUMMARY OF THE INVENTION

[0011]The present invention provides improved methods for purifying
glycerin contaminated with one or more lower boiling alcohols such as
methanol, ethanol, straight, branched or cyclic C3-C6 alcohols, and the
like. The methods are particularly useful for purifying crude glycerin
phases recovered from the synthesis of biofuels.

[0012]The present invention uses distillation techniques to strip alcohol
contaminants from glycerin. In contrast to conventional methods that
carry out distillation either under substantially anhydrous or very wet
conditions, the present invention carries out distillation in the
presence of a limited amount of water, e.g., from about 0.8 to about 5
parts by weight of water per 100 parts by weight of contaminated glycerin
to be purified.

[0013]Several advantages result. Firstly, even though only a small amount
of water is added, the addition drops the bottoms temperature
significantly. For instance, when separating methanol from glycerin,
using a limited amount of water allows the bottoms temperature to be
under 200° C. and even under 190° C. at ambient pressure.
This lower temperature as well as the impact of the water upon
glycerin/polyglycerin equilibrium inhibits polyglycerin formation. The
distillation is relatively easy due to the minimal amount of water that
is present. The energy savings and throughput gains via improved reflux
ratios are considerable. In short, using a limited amount of water avoids
the major drawbacks associated with anhydrous distillation.

[0014]Using a limited amount of water also avoids the major drawbacks
associated with wetter distillations. When only a limited amount of water
is present, the soap remains soluble in the glycerin. Consequently, the
small amount of water generates very little and even no foaming. The
small amount of foaming that might be observed is easily handled with the
addition of moderate amounts of anti-foaming agents, which is not the
case with wetter distillations. Additionally, no salt deposits or
reboiler plugging have been observed in the practice of many embodiments.
Further, since corrosive acid need not be added to lower pH to control
foaming, the distillation can occur in economical equipment such as that
fabricated from mild/carbon steel. More expensive metallurgy is not
required. The small amount of water also reduces the glycerin viscosity
enough so that a subsequent phase separation between a glycerin phase and
a FAME/FFA phase post-distillation, after acidification takes place with
a sufficiently fast rate and completion. In many embodiments, the
glycerin finally produced by this process typically has less than 1% of
organic materials.

[0015]The performance is excellent. In representative modes in which
methanol is stripped from crude glycerin recovered from biofuel
synthesis, the crude glycerin has been assessed to include less than 500
ppm methanol after distillation. The stripped methanol is also highly
pure, allowing it to be recycled.

[0016]After the methanol stripping, crude glycerin is easily separated
from soap and fatty acid ester by an aqueous acid wash in representative
embodiments. The wash yields an aqueous phase containing highly pure
glycerin with low organic contaminant content and an organic phase with
fatty acid/fatty acid ester that can be recycled for further processing
or use, such as further biofuel synthesis.

[0017]In one aspect, the present invention relates to a method of
purifying crude glycerin. An alkaline admixture (preferably one that is
substantially anhydrous) comprising glycerin, soap, a fatty acid ester,
and at least one other alcohol is provided. The other alcohol has a lower
boiling point than glycerin. A sufficient amount of water is added to the
admixture so that the admixture after adding the water includes from
about 0.8 to about 5 parts by weight water per 100 parts by weight of the
admixture. After adding the water, the admixture is distilled under
conditions effective to strip away substantially all of the other
alcohol. After stripping the other alcohol, the pH of the admixture is
lowered with aqueous acid under conditions effective to convert the soap
to free fatty acid and to form a first organic phase comprising the free
fatty acid and the fatty acid ester and a second aqueous phase comprising
the glycerin. The organic and aqueous phases are separated.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]The above mentioned and other advantages of the present invention,
and the manner of attaining them, will become more apparent and the
invention itself will be better understood by reference to the following
description of the embodiments of the invention taken in conjunction with
the accompanying drawings, wherein:

[0019]FIG. 1 is a schematic flow diagram showing how purification
principles of the present invention are incorporated into a process for
synthesizing a biofuel via the transesterification of triglycerides with
methanol; and

[0020]FIG. 2 is a schematic flow diagram showing how crude glycerin
recovered in the biofuel synthesis process of FIG. 1 is purified using
principles of the present invention.

DETAILED DESCRIPTION OF PRESENTLY PREFERRED EMBODIMENTS

[0021]The embodiments of the present invention described below are not
intended to be exhaustive or to limit the invention to the precise forms
disclosed in the following detailed description. Rather the embodiments
are chosen and described so that others skilled in the art may appreciate
and understand the principles and practices of the present invention.

[0022]The present invention provides methodologies for separating glycerin
(also known as glycerol) from contaminants including one or more other
alcohol(s), at least one soap, and at least one fatty acid ester. The
purification desirably occurs in two or more stages. In a first stage,
the glycerin is separated from the one or more alcohols via distillation.
Then, the glycerin is separated from the other contaminants in one or
more additional stages. Because distillation techniques are desirably
used to accomplish the removal of the one or more other alcohols, and
because glycerin has a relatively high boiling point of about 290°
C., the methodologies are particularly advantageously applied to alcohol
contaminants having respective boiling points that are at least
30° C., desirably at least 60° C., and more desirably at
least about 100° C. less than that of glycerin. These other
alcohols may include one or more hydroxyl groups. The alcohol
contaminants may be linear, branched, cyclic, fused, spyro, combinations
of these, and/or the like. The one or more hydroxyl groups may be
primary, secondary, or tertiary. In illustrative embodiments, alcohol
contaminants include one or more of methanol, ethanol; linear or branched
C3 alcohols; linear or branched C4 alcohols, linear or branched C5
alcohols, and/or the like. Methanol, ethanol, and/or C3 alcohols, often
methanol, are contaminants encountered in crude glycerin resulting as a
by-product in the synthesis of biofuels, as discussed further below.

[0023]The relative amount of alcohol contaminant(s) that can be removed
from crude glycerin using techniques of the present invention can vary
over a wide range, demonstrating the wide applicability of the present
invention. In many representative, practical applications, the weight
ratio of glycerin to other alcohol contamination may be in the range from
about 1000:1 to 1:1000, sometimes 100:1 to 1:100; or sometimes 10:1 to
1:10. The invention can be applied to purify glycerin when the content of
other alcohol contaminant(s) is outside these typical ranges, but these
tend to be ranges that would be encountered most likely in actual
practice. For example, crude glycerin resulting as a by-product from
biofuel synthesis might have from about 10 to 90, 20 to 70, and sometimes
30 to 60 weight percent other alcohol(s), depending upon the amount of
excess alcohol used as a reactant in the synthesis reaction. The
purification techniques of the invention allow not only the glycerin, but
also the other alcohol(s) to be recovered with high purity.

[0024]According to preferred modes of practice, glycerin is distillingly
separated from one or more, lower boiling alcohol contaminants in the
presence of from about 0.8 to about 5, preferably from about 1 to about
4.5, more preferably from about 1.5 to about 4 parts by weight water is
added per 100 parts by weight of the crude glycerin (not including the
added water, but including other liquid phase contaminants that might be
present in the crude glycerin such as soap(s), catalyst, spent catalyst,
fatty acid ester, glycerides, and the like). The presence of a limited
amount of water at the time of alcohol stripping is distinguished from
conventional distillation procedures that occur under substantially more
anhydrous or substantially more wet conditions. Whereas wet stripping
tends to be difficult to due excessive foaming caused by soap
contaminants that are typically present following biofuel synthesis, any
soap tends has a greater tendency to stay dissolved in the glycerin when
the water content of the distillation feed is limited. This dramatically
reduces foaming and avoids having to resort to corrosive acid chemistries
and expensive metallurgy to deal with foaming. If any foaming were to
occur, it is so minimal as to be easily handled by adding only a minor
amount of an anti-foaming agent such as Antifoam 2210 commercially
available from Dow Corning. Using limited water also avoids the problems
associated with reboiler plugging that follow from wetter distillations.
The need to remove excessive amounts of water to further purify the
glycerin is also entirely avoided.

[0025]On the other hand, using a limited amount of water also avoids the
problems associated with substantially anhydrous stripping of alcohol
contaminants from glycerin. Under substantially anhydrous conditions,
distillation will tend to occur at higher temperatures, such as over
200° C. or even over 210° C. under ambient pressure and
basic pH conditions. Under these conditions, substantial amounts of
glycerin are converted to polyglycerin by-product, lowering the yield of
glycerin recovery. The viscosity of the distillation bottoms also tends
to be higher than might be desirable for easy handling. In contrast, with
a limited amount of water, the alcohol stripping can more easily occur
below 200° C., desirably below 190° C., even at a
temperature of 170° C. to 180° C., under ambient pressure,
when stripping an alcohol such as methanol from glycerin. In addition to
lowering the boiling temperature, which inhibits polyglycerin formation,
the presence of water also inhibits polyglycerin formation due to
equilibrium effects. Generally, the distillation feed needs to be
substantially anhydrous for polyglycerin to form. Even at low content
according to the present invention, the water plays an important role as
an intermediate boiling compound to ease the purification and avoid the
production of polyglycerin by-products.

[0026]Significant other advantages result as well. The viscosity of the
bottoms is dramatically lowered. Additionally, the reflux ratio of the
distillation column drops considerably, improving both energy savings as
well as throughput. For example, when stripping methanol from glycerin in
the presence of 2 weight percent water leaving only 500 ppm methanol in
the bottom in a column with 20 theoretical trays, the reflux ratio drops
from 1.75:1 to 1.25:1. This improves energy savings and throughput by 20%
for a given distillation unit. Any soap impurity, such as might be
present from biofuel synthesis, remains soluble in the glycerin. De
minimis if any salt deposits or reboiler plugging are observed. If a
small amount of foaming does occur, it can easily be controlled by adding
a small amount of an anti-foaming agent. This avoids antifoaming efforts
that rely upon corrosive acid chemistries. As a consequence, distillation
can occur in distillation equipment having ordinary metallurgy
characteristics such as mild carbon steel as opposed to being limited
only to more expensive, corrosion-resistant equipment.

[0027]Many alcohols other than methanol may tend to form azeotropes with
water. Consequently, both water and alcohol may be removed when
distillation occurs in the presence of such azeotropes. Under such
circumstances, the azeotropic water that is removed or to be removed may
be replenished in advance and/or as the water is taken out. At least a
portion of this extra water could be added to the feed stream right away
prior to distillation and/or a portion could be added at a suitable site
such as to the reboiler or to the lower part of the distillation
equipment being used.

[0028]The distillation may occur under a wide range of pressures including
vacuum, partial vacuum, ambient pressure, or elevated pressure. On a
commercial scale, the distillation occurs very effectively under ambient
pressure, which is economical as well. Consequently, it can be
appreciated that the distillation can be carried out in a wide range of
distillation equipment, including those configured for batch or
continuous processes.

[0029]The residual amount of alcohol contamination in the glycerin can be
quite low. Distillation desirably is carried out under conditions so that
the content of alcohol contaminant(s) remaining in the bottoms is less
than 0.2%, desirably less than 0.1%, and more desirably less than 500
ppm. The stripped alcohol also desirably is highly pure as well. In
typical embodiments, the stripped alcohol includes less than 500 ppm
glycerin. Methanol, which does not form an azeotrope with water, includes
less than 1000 ppm, preferably less than 500 ppm, more preferably less
than 300 ppm water on a weight basis.

[0030]The principles of the present invention can be incorporated into
more complex purification systems in which glycerin is separated from a
combination of contaminants including not only one or more other alcohols
but also other materials such as fatty acids, fatty acids esters, soaps,
catalysts, spent catalysts, combinations of these and the like. For
purposes of illustration, these aspects of the invention will be
described in the context of purifying crude glycerin generated as a
by-product in the synthesis of biofuels. Such crude glycerin results as a
by-product of the transesterification of glycerides with an alcohol.

[0031]In a typical transesterification process, a glyceride is reacted
with excess alcohol in the presence of a base catalyst. Vegetable oils,
nut oils, animal fats, seed oils, fish oils, and the like are examples of
suitable feed stocks containing glycerides. Examples of oils include
rapeseed oil, sunflower oil, safflower oil, soybean oil, coconut oil,
gourd oil, corn oil, cottonseed oil, canola oil, olive oil, palm oil,
peanut oil, sesame oil, almond oil, cashew oil, hazelnut oil, macadamia
oil, pecan oil, pistachio oil, walnut oil, tung oil, castor oil, coconut
oil, hemp oil, mustard oil, combinations of these, and the like. These
oils often include triglycerides, but also may contain some amount of
monoglyceride and diglyceride. The glycerides are mono, di, and tri
esters of glycerin with a fatty acid. Some free fatty acid may also be
present in these oils, but suppliers often reduce the fatty acid content
by stripping to less than 1%, or even less than 0.5% by weight. Lower
fatty acid content is desirable. Alternatively, oils with higher amounts
of fatty acid like Yellow Grease (recycled oil from restaurants) or
Jatropha oil, as well as recycle methylester from the biodiesel synthesis
process, are often pre-esterified with glycerin or the alcohol to be used
in a transesterification.

[0032]The alcohol reactant can be selected from any one or more alcohols
that undergo alcoholysis substitution exchange reaction with the
glyceride material to transesterify the reactants into glycerin and fatty
acid esters. Representative examples of alcohols include methanol,
ethanol, propyl alcohol, isopropyl alcohol, linear and branched C4
alcohols, and the like. Methanol is often preferred in this reaction to
produce a fatty acid methyl ester (FAME) product.

[0033]The stoichiometric excess of alcohol used can vary over a wide
range. As general guidelines, it is desirable to use enough of the
alcohol so that the reaction products phase separate into two liquid
layers. Often, using 170% to 300%, more desirably about 200% of the
theoretical, stoichiometric amount of alcohol can be used for reactions
carried out at suitable temperatures, such as from about 15° C. to
being heated up under pressure to way above 100° C. in order to
get the reaction done in just a few seconds or even faster. This
generally may correspond to using from about 7 to about 40 weight percent
of alcohol based upon the weight of the oil used.

[0034]The transesterification reaction is catalyzed by bases such as
potassium hydroxide, sodium hydroxide, sodium methoxide, potassium
methoxide, sodium ethoxide, sodium methylate, potassium methylate, sodium
ethylate, potassium ethylate, combinations of these, and the like. The
catalyst may be used in any amount effective to catalytically facilitate
the transesterification reaction at least to some degree. In typical
embodiments, using from about 0.1 to about 2 weight percent of catalyst
based upon the weight of the oil would be suitable. Prior to introducing
the alcohol to the reaction vessel, the catalyst is often pre-mixed with
the alcohol.

[0035]Thus, the transesterification reaction desirably occurs by mixing
the feedstock of glyceride and the pre-mix of alcohol and catalyst in a
suitable reaction equipment for a suitable time period. If the reactor
has a headspace, this may optionally be blanketed with an inert gas such
as nitrogen, argon, carbon dioxide, or the like. Carrying out the
reaction in clean dry air is possible, but oxygen may cause peroxides and
other undesirable oxygenated products to form, specifically with the
double bonds that are present in many unsaturated oil compounds.

[0036]However, the reaction desirably occurs under as anhydrous conditions
as is practically feasible. Desirably, the water content of the mix is
less than 0.1 weight percent, and more desirably under 0.05 weight
percent. If desired, the reaction can occur under pressure or under
vacuum, although ambient pressure is suitable as well.

[0037]As the reaction proceeds, the contents of the reactor will tend to
phase separate into two liquid layers. The top layer is a FAME-rich
layer, while the bottom layer is a glycerin-rich layer. In addition to
FAME, the FAME-rich layer will also include some glycerin, left-over
alcohol, soap (spent catalyst as well as soap resulting from free fatty
acid in the oil feedstock, for instance), catalyst, and sometimes a very
minor amount of water. The glycerin-rich phase, also referred to as crude
glycerin, also will include some FAME, left-over alcohol, soap (spent
catalyst as well as soap resulting from free fatty acid in the oil
feedstock, for instance), catalyst, and sometimes a very minor amount of
water. The glycerin-rich phase can be separated from the FAME-rich phase
by any suitable method, such as decanting, and then purified in
accordance with principles of the present invention. One example of a
purification system for purifying the crude glycerin will be described
below in connection with FIGS. 1 and 2.

[0038]Although the transesterification reaction is an equilibrium
reaction, the fact that the reaction product glycerin tends to separate
into a separate phase helps to drive the equilibrium toward completion
since the products of the reaction are removed from the reaction
environment as a practical matter due to the phase separation. However,
because the catalyst generally prefers to be in the crude glycerin phase,
the reaction will tend to slow down as more glycerin is made.
Accordingly, it may be desirable to carry out the reaction in two or more
stages. In a first stage, the reaction is allowed to proceed until the
rate slows down too much so that a desired residence time would be
exceeded. The crude glycerin phase is then decanted or otherwise removed
from the reaction stream. Fresh catalyst is added to the reactor,
optionally pre-mixed with additional alcohol reactant, and the reaction
is allowed to proceed further. After a time period, the additional crude
glycerin is removed from the reactor and may be combined with the crude
glycerin from the first stage for purification treatment. Often, two
stages are sufficient for the reaction to proceed very far to completion,
e.g., 99% or more, but one or more additional stages can be performed if
further reaction progress is desired.

[0039]The reaction time will depend upon factors including the
temperature, pressure, amount of catalyst, amount of alcohol relative to
the oil, intensity of initial mixing and the like.

[0040]The reactor may include features that allow the reactor contents to
be mixed or otherwise agitated during the course of the reaction. The
reactor may also be fitted with features, such as a jacket or the like,
that allows the reactor to be heated or cooled to desired temperature(s).

[0041]Conditions for carrying out the transesterification reaction are
described in the patent and technical literature, including documents
such as U.S. Pat. Nos. 5,424,467; 6,262,285; 6,174,501; 7,126,032; and
7,138,536; as well as JP 10218810; and also Peterson, C. L., Feldman, M.,
Korus, R., and Auld, D. L. (1991), "Batch Type Transesterification
Process for Winter Rape Oils" Applied Engineering in Agriculture, 7(6)
pp. 711-716 and "Process Development of Rapeseed Oil Ethyl Ester as a
Diesel Fuel Substitute" M. S. Thesis by Narendra Bam, University of
Idaho, July, 1991.

[0042]The crude glycerin collected from the one or more stages of reaction
advantageously may be purified using procedures of the present invention.
In a first purification step, the alcohol reactant, often methanol or
ethanol, is stripped from the crude glycerin using distillation
techniques as described above. Because the crude glycerin collected from
the transesterification step tends to be substantially anhydrous,
typically including less than about 0.5 weight percent water, enough
water is added to help ensure that the distillation can occur at a
temperature below 200° C., desirably below 190° C. and even
below 180° C. Preferably, enough water is added so that the
distillation feed includes 0.8 to about 5, preferably from about 1 to
about 4.5, more preferably from about 1.5 to about 4 parts by weight
water per 100 parts by weight of the crude glycerin (not including the
added water) being distilled for the reasons discussed above.

[0043]In carrying out this distillation, the pH is preferably sufficiently
high to ensure that the soap content of the crude glycerin is soluble in
the glycerin and remains in the form of soap. If the pH were to be too
low, corrosion becomes a main concern. As collected from the
transesterification reaction, the pH of the glycerin will be at a
suitable alkaline pH to carry out the distillation, e.g., a pH of about
10 or more, even about 12 in some embodiments. Optionally, although not
required, the pH can be lowered prior to distillation to make the crude
glycerin less alkaline (e.g., 8 to 10), more neutral (e.g., 7 to 8) or
even moderately acidic so long as the soap is not converted to an
insoluble fatty acid ester in a way that would unduly compromise the
effectiveness of the alcohol stripping.

[0044]Distillation is carried out until the alcohol contaminant level in
the crude glycerin is reduced to desired levels within practical limits.
By way of example, one mode of practice can reduce the methanol content
in crude glycerin to less than 0.2 weight percent, even less than 500
ppm, based upon the total weight of the crude glycerin (including the
limited amount of water in the crude glycerin for this calculation) from
a crude glycerin including one weight percent water at the start of the
distillation. This distillation was carried out at about 180° C.
at 760 mmHg and is described further in the examples below. The stripped
alcohol also tends to be sufficiently pure that it may be recycled and
used for desired purposes, including being recycled for participation in
further biofuel transesterification reactions.

[0045]After the distillation, the remaining crude glycerin will still
include contaminants including soap, fatty acid ester(s), catalyst, and
the limited amount of water added to ease the distillation phase of the
purification. The next phase of the purification separates the glycerin
from these other organic contaminants, any of which also can be recycled
optionally after further purification or other handling. One way to
accomplish the separation of the glycerin from the other organics is to
wash the crude glycerin with an acidic water wash. This converts the
soap(s) to free fatty acid(s). The mixture also phase separates into a an
aqueous, glycerin-rich phase with very low organic contaminant content
and a separate organic phase containing organic materials including the
fatty acid(s), fatty acid ester(s), and the like. In practical effect,
the acid water wash incorporates a chemical reaction that converts some
contaminants to a form more amenable to separation from the glycerin as
well as a liquid-liquid extraction to isolate the glycerin from other
organic constituents of the crude glycerin. Note with this approach that
the addition of water and acid is delayed until after the excess alcohol
reactant(s) in the crude glycerin have been stripped out via the prior
distillation phase of purification.

[0046]The pH desirably is lowered sufficiently to convert the soap content
to fatty acids. In many embodiments, lowering the pH to a range from
about 3 to about 5, preferably about 3 to about 4, is suitable. Lower pH
targets could be used, but this would require more acid than is required
to achieve the desired goal of converting the soap to fatty acids. A wide
range of acids can be used to lower the pH to the desired range. Strong
mineral acids such as HCl or H2SO4 or strong organic acids such
as citric acid are preferred. Using either concentrated or dilute forms
of these acids would be suitable. However, when the amount of water that
can be present in the glycerin-rich phase is subject to a specification
that limits the amount of water in that phase (e.g., one specification
might specify a maximum of 18 weight percent water in the phase based
upon the total phase weight), using a concentrated acid is often
desirable to avoid a risk of adding to much water. It follows that any
amount of water may be added as desired, subject to satisfying water
specifications that might be applicable. When a water specification
applies, the amount of water added is calculated so that the
glycerin-rich phase will be within the specification after phase
separation.

[0047]Either acid or water may be added to the crude glycerin, or the two
ingredients can be added together. For commercial scale processes, it is
desirable to add the water first, and then the acid. The admixture
desirably is mixed throughout acid addition for a sufficient period of
time at suitable temperature(s) to allow the conversion of soap to fatty
acid to occur. By way of example, the admixture is mixed for a time
period in the range from about 3 minutes to 72 hours, preferably 30
minutes to 48 hours, more preferably 1 hour to 24 hours at a temperature
in the range from above about 0° C. to about 95° C.,
preferably about 20° C. to about 70° C., more preferably
about 35° C. to about 60° C. In one embodiment, the
reaction took place at 50° C. for about 18 hours. When the
admixture is allowed to settle after the reaction, a top organic layer
containing fatty acid esters and free fatty acid separates relatively
quickly from a bottom, aqueous, glycerin-rich layer in the presence of
the added water. After the layers are separated, the pH of the glycerin
layer may be raised so as to be mildly acidic (e.g., from about 5 to
about 6) or neutralized (from about 6 to about 8) by addition of a
suitable base such as NaOH, KOH, and/or the like. The material in the
upper layer may be recycled such as being recycled for conversion to
biofuel.

[0048]The resultant glycerin may be highly pure with respect to organic
contaminants. Embodiments have yielded glycerin at this stage in which
the organic content is less than 1 weight percent and even less than 0.5
weight percent based upon the total weight of the contained glycerin.
This indicates that the stripping of alcohol contaminants first in the
presence of limited water, followed by the acidic water wash removes
contaminants including other alcohols, fatty acid, and fatty acid ester
from glycerin very effectively.

[0049]FIGS. 1 and 2 schematically show an illustrative biofuel synthesis
scheme 10 incorporating a glycerin purification system 12 of the present
invention. Referring first mainly to FIG. 1, the reactants used to carry
out the transesterification reaction, shown occurring in two stages 14
and 16 are fed to a suitable reactor vessel. These reactants include a
fresh supply of alcohol and catalyst from source 18 as well as one or
more oil feed stocks from source(s) 20. For purposes of illustration,
source 18 provides a premix of methanol and catalyst, while source 20
provides a low FFA (free fatty acid content less than 1 weight percent,
even less than 0.5 weight percent) vegetable oil. Additionally, recycled
methanol and catalyst recovered by system 12 is also fed to the reactor
vessel from supply 22. Recycled fatty acid ester recovered by system 12
may also be fed to the reactor vessel from supply 24. The recycled
supplies 22 and 24 may be fed to the reactor vessel to supply either the
first reaction stage 14 or the second reaction stage 16. For purposes of
illustration, the recycled methanol and catalyst from supply 22 is added
to the reactor vessel to carry out the second reaction stage 16 after the
first reaction stage is complete, while the recycled fatty acid ester
from supply 24 is added to the reactor vessel as part of the feed to
carry out the first reaction stage 14.

[0050]When the reactants are fed to the reactor vessel, the reactor
contents tend to phase separate into a fatty acid ester rich layer and a
crude glycerin layer as the transesterification reaction proceeds. As
mentioned above, however, the catalyst tends to be more soluble in the
glycerin phase than in the fatty acid ester phase, which also tends to
include most of the triglyceride material. Thus, reaction tends to slow
down as more crude glycerin forms. Accordingly, to conclude the first
reaction stage 14, the crude glycerin is decanted from the reactor vessel
as shown by step 26. Fresh methanol and catalyst from supply 22 are then
added to carry out the second reaction stage 16. When this reaction is
complete, the decanted glycerin of step 26 is used to wash the reactor
contents at step 28. As a consequence of this wash, the reactor contents
phase separate into a fatty acid ester rich layer and a crude glycerin
layer. The fatty acid ester rich layer, for which the fatty acid ester
will be used as a constituent of biodiesel, is carried forward to step
100 for purification, as will be described further below. The crude
glycerin layer resulting from the wash is decanted at step 30 (shown on
both FIGS. 1 and 2) and then carried forward to purification system 12.

[0051]Purification system 12 is shown in more detail in FIG. 2. As a first
step, the decanted glycerin from step 30 is combined with a limited
amount of water so that the resulting admixture includes from about 0.8
to about 5, preferably from about 1 to about 4.5, more preferably from
about 1.5 to about 4 weight percent water based upon the weight of the
decanted crude glycerin to which the water is to be added. The water can
come from a fresh source. Alternatively, as shown, the water can come
from the aqueous layer that phase separates in the acid water wash of
step 34. The resulting decanted, crude glycerin including the limited
amount of water is then subjected to distillation in step 32 in order to
strip the methanol. The resultant dry methanol 36 can be recycled as
shown and used as a source for the methanol and catalyst supply 22 (see
FIG. 1).

[0052]In the meantime, decanted aqueous phase from decanting step 102 of
FIG. 1 is combined with a mineral acid from source 38 of FIG. 2 to carry
out the phase separation in step 34. The decanted aqueous phase of step
102 prior to the addition of this acid is highly alkaline and will tend
to include not only water, but also glycerin, methanol, soap, and fatty
acid ester (FAME). It is desirable to remove the soap (which is converted
to free fatty acid, FFA) and FAME from this decanted water before using
the water further. To accomplish this, enough of the mineral acid is
added to lower the pH to a range at which the soap is converted to free
fatty acid. In step 34, the admixture then separates into an aqueous
phase including mainly water, glycerin, and methanol and an organic phase
including mainly free fatty acid and fatty acid ester, specifically FAME.
The organic layer can be used for recycle, being a source of feedstock
for supply 24 of FIG. 1.

[0053]The aqueous phase resulting from step 34 is subjected to a
distillation in step 40 to separate the methanol from the water and
glycerin. The methanol resulting from this distillation is combined with
the methanol recovered from the distillation of step 32. The water with a
minor amount of glycerin is recycled in two ways. As shown by step 42, a
portion of the water with glycerin is used in supply 104 to carry out a
water wash of the biodiesel in step 100 (FIG. 1), while another portion
is used along with mineral acid to subject the crude glycerin from
distillation step 32 to an acid water wash. Thus, mineral acid from
supply 44, the crude glycerin from the distillation step 32, and the
water with glycerin from step 42 are combined to conduct an acid water
wash of the crude glycerin in step 46. The acid water wash is carried out
according to the protocols described above. An exemplary embodiment is
also described in the Examples, below. In step 46, the soap in the crude
glycerin is converted to free fatty acid (FFA). The FFA and the FAME
separate into an organic, top phase, while the water and glycerin
separate into a bottom, aqueous phase. The purified, aqueous glycerin
resulting from step 46 can be sold, further purified to separate the
glycerin from water, or otherwise processed or handled as desired. The
FFA/FAME layer can be used as a source of feedstock for supply 24 (FIG.
1).

[0054]Referring again to FIG. 1, the purification of the biofuel resulting
from step 28 will now be described in more detail. In step 100, the
biofuel obtained from step 28 is washed with water. The water is obtained
from supply 104, using recycled water from step 42 as a feedstock. The
admixture of step 100 separates into an organic phase containing purified
FAME (e.g., 98% pure in illustrative embodiments) and an aqueous phase.
The aqueous phase is decanted in step 102 and used as the source water
for step 34 in FIG. 2. The water from supply 104 is mildly acidic, but
the overall admixture in step 100 will be alkaline due to the strong
alkalinity of the biofuel obtained from step 28.

[0055]Next, the FAME phase from step 100 is subjected to an aqueous acid
wash in step 106. This wash helps to remove metal impurities as well as
converting residual soap into tolerable amounts of fatty acid. Excess
strong acid remains in the aqueous layer, which is decanted or otherwise
separated, e.g., by centrifuge. The aqueous acid is obtained from source
108. A wide range of acids may be used. Strong mineral or organic acids
such as HCl, H2SO4, citric acid, phosphoric acid, combinations
of these, and the like may be used in dilute or concentrated form. The
resultant admixture separates into an organic phase include the FAME and
an aqueous phase. The aqueous phase can be recycled and used as part of
the supply 104. The FAME phase can be further processed or otherwise
handled in one or more subsequent steps. For purposes of illustration,
the FAME is next dried in step 110, and then is ready for use or
distribution.

[0056]The present invention will now be described with reference to the
following illustrative examples.

Example 1

[0057]This example uses principles of the present invention to remove MeOH
from crude glycerin using distillation in the presence of a limited
amount of water. The crude glycerin treated in this example was the
glycerin bottom obtained from the transesterification of soybean oil with
methanol. This crude glycerin had the following characteristics:

[0058]The limited amount of water makes the methanol removal easier
(compared to a wetter glycerin distillation by avoiding excessive foaming
(foaming requires water). Additionally, if too much water were to have
been present during distillation the soap present in the crude glycerin
could cause coking and plugging around the reboiler.

[0059]Continuous distillation was used to strip the methanol from the
crude glycerin. For purposes of this example, it was desired that the
methanol content in the purified glycerin be no more than 0.2% by weight.
As preparation for the distillation, ChemCAD simulation indicated that in
order to meet glycerin's max 0.2% MeOH spec the bottom temperature would
be >220° C. at 760 mmHg if the distillation were to be carried
out under substantially anhydrous conditions. However, this temperature
would be high enough to create an undue risk of significant polyglycerin
formation or other decomposition reactions. However, by adding 1% water
to the feed (based upon the total weight of the crude glycerin), the
simulation showed the bottom temperature could be lowered to
˜180° C. at 760 mmHg and still produce the bottom with MeOH
in the desired specification. Therefore, 1% DI water was added to the
crude glycerin for this example. A moderate head reflux was applied so
the MeOH could meet the water spec. The following tables summarize the
distillation parameters:

[0060]With 1% water added to the feed, the bottom temperature was kept at
about 190° C., well below 200° C. The recovered MeOH
included only 0.02 weight percent water and, thus, could be recycled for
biodiesel transesterification. There was no precipitation or coking
observed in the reboiler. No soap precipitated out at room temp.,
however, the material was quite viscous. On a commercial scale, it might
be good to add water to this bottom stream right at the after cooler to
lower the viscosity and ease handling. Some foaming was observed in the
reboiler. Dow coming Antifoam 2210 worked very well to break the foam.

Example 2

[0061]This example describes further purification of the crude glycerin
processed in accordance with Example 1. After the MeOH was removed from
the crude glycerin, the glycerin had a pH of 12 and still contained soap
and methyl esters. By acidifying the crude glycerin, the soap could be
converted into FFA. The FFA and FAME (fatty acid methyl ester) could be
separated from the glycerin and recycled. For this example, a
specification was applied in which the recovered, purified glycerin would
include a maximum of 18 weight percent water based on the total weight of
glycerin and water.

[0062]In order to convert the sodium salt of fatty acids (i.e., the soap)
to free fatty acid, the crude glycerin acidified to pH 3-4.
H2SO4 was added slowly into the glycerin until pH ˜3.5.
Overall, 2.7 weight percent H2SO4 (98%) was used. After the
acid was added, 14% by weight water was added as well based on the weight
of the crude glycerin. The reaction took place at 50° C. for
˜18 hours. The amount of water used was calculated so that the
glycerin bottom, after FFA and FAME separation, would meet the max 18%
water specification. In this example, acid was added first, followed by
addition of water. The order of this addition can be reversed. For
example, the water may be added first to reduce viscosity before the acid
is added. However, if the addition of water initiates foaming in the
mixing process, then the acid desirably is added first to convert the
foam-causing soap into fatty acid. It was observed that the consistency
of the acidified glycerin changed from viscous fluid at the beginning to
gel like material and to liquid again after fatty acid was formed. Due to
the presence of water, layers developed with an upper FAME/FFA layer and
a lower water/glycerin layer. The composition of each of the layers was
analyzed as follows:

[0063]Analysis showed that the glycerin layer included less than 0.3
weight percent of other organics. This is well within a typical industry
specification such as one that requires a maximum of 1% by weight of
other organics (sometimes expressed as a maximum of 1% M.O.N.G., wherein
the term "M.O.N.G." is an abbreviation for matters organic non-glycerin,
an English translation of a French expression). This indicates, that the
removal of methanol, followed by water/acid addition removes methanol and
then the FFA/FAME quite efficiently. At end, the glycerin pH was adjusted
to 6 by NaOH addition.

[0064]In a follow up, the upper layer of FAME/FFA was successfully
recycled and converted to biodiesel.

Example 3

[0065]A limited amount of water is added to a crude glycerin feedstock
obtained from the synthesis of biodiesel having to provide an admixture
having the following range of characteristics:

[0066]The admixture is then distilled to strip methanol. The distillation
is done in either mild steel or stainless steel units of 3-6 ft diameter
at either atmospheric pressure or moderate overpressure or vacuum. Feed
enters the column at or below the middle of the distillation tower. At
the head, a Methanol stream with 500 ppm water is recovered. This dry
methanol is fit for reuse in a biodiesel synthesis process.

[0067]The bottom temperature ranges between 160° C. and 190°
C. depending mostly upon the amount of added water. A glycerin stream is
taken from the bottom. This stream contained a maximum of 0.5% by weight
Methanol. Despite the high amount of soap present, the stream is
substantially homogenous. The glycerin is acidified to a pH of 2-4 while
the stream is still warm to allow the quick removal of a FFA/FAME layer
by decantation or centrifugation. No reboiler fouling, coking or
decomposition resulted from the distillation. Very moderate foaming was
kept under control by an antifoam agent (Dow Corning 2210) injection to
the feedstock as needed.

[0068]Other embodiments of this invention will be apparent to those
skilled in the art upon consideration of this specification or from
practice of the invention disclosed herein. Various omissions,
modifications, and changes to the principles and embodiments described
herein may be made by one skilled in the art without departing from the
true scope and spirit of the invention which is indicated by the
following claims.